In recent years, the use of mobile telephones has increased dramatically due to the obvious convenience and benefits associated with such use, as well as the increased availability and affordability of mobile telephone service. This increased availability and affordability of mobile telephone service is due, at least in part, to the adoption of digital cellular standards and the use of low cost digital electronic components. However, some problems have arisen as a result of the adoption of these standards and the use of these components.
For example, people who wear hearing aids are finding that the T-coil mode of operation of a hearing aid is unacceptably noisy when operating a digital cellular telephone. This noise is a result of low frequency baseband interference from the digital cellular telephone coupling into the hearing aid, thereby masking the audio signals emanating from the audio signal generator in the earpiece of the digital cellular telephone. More particularly, this noise is a result of low frequency digital switching currents flowing within the battery of the digital cellular telephone. These low frequency digital switching currents generate corresponding low frequency magnetic fields which couple into the hearing aid and cause the masking of the audio signals emanating from the audio signal generator in the earpiece of the digital cellular telephone. These low frequency digital switching currents, and the corresponding generated low frequency magnetic fields, are particularly bothersome in areas surrounding the battery of the digital cellular telephone since this is where these currents and fields are at their greatest magnitude as all supply and return currents for the electronic circuitry of the digital cellular telephone originate from and return to the battery, respectively.
The above-described low frequency baseband interference is a relatively new problem which has come onto the scene as a result of the introduction of digital cellular telephones. Normal hearing persons have no perception of the low frequency magnetic fields propagating within and around a digital cellular telephone unit. However, hearing impaired persons are well aware of these low frequency magnetic fields as their hearing aids respond quite well to these rouge magnetic fields, making the digital cellular telephone unit essentially unusable when a hearing aid is operating in the T-coil mode of operation.
In order to allow universal use of digital cellular telephones and accommodate the needs of hearing impaired persons, both private digital cellular telephone manufacturers and government regulatory entities have been in search of a solution to the above-described interference noise problem. One proposed solution to this interference noise problem involves placing an electrically conductive shield around the entire digital cellular telephone unit so as to reduce any unwanted emissions. However, this solution can be costly and really only reduces high frequency emissions (e.g., due to skin effect), not the relatively low frequency emissions associated with digital switching currents. Thus, a need still remains for a technique for reducing low frequency baseband interference noise in digital cellular telephones and the like.
In view of the foregoing, it would be desirable to provide a technique for reducing low frequency baseband interference noise in digital cellular telephones which overcomes the inadequacies and shortcomings of the above described and other proposed solutions. More particularly, it would be desirable to provide a technique for reducing low frequency baseband interference noise in digital cellular telephones and the like in an efficient and cost effective manner.